Split-field microscope

ABSTRACT

The split-field microscope of the present invention has a polarizer in the illuminator, a polarizing beam splitter for simultaneous or alternative illumination of both object fields, equal optical path lengths for the polarized split partial beams between splitting and reunion, means for deflecting the polarized partial beams in the direction toward parallel-disposed objectives and a quarter-wave plate disposed in each illuminating beam path. The imaging ray beams are passed backwards through the aforementioned components up to the central prism and a tube lens system and an ocular, connected after the central prism, are transited by the combined imaging beams.

i United States atet 1 1 Hock et a1.

1 Jan. 15, 1974 1 4] SPLIT-FIELD MICROSCOPE [75] Inventors: FromundHock, Wetzlar', Hennig Feldmann, Muenchholzhausen; Heinz Fialkowslri,Wetzlar, all of Germany [73] Assignee: 1E. Leitz GmlbH, Wetzlar, Germany[22] Filed: Oct. 12, 1971 [21] Appl. No.: 188,307

[30] Foreign Application Priority Data Oct. 19, 1970 Germany P 20 51174.4

[52] US. Cl 350/15, 350/30, 356/168 [511' lint. C1. G02b 21/11 8 [58]Field of Search 350E235, 30, 35, 561 147,

UNITED STATES PATENTS 3,523,736 8/1970 Bottomleyn. 356/156 2,732,7601/1956 Rantsch 356/168 3,481,673 12/1969 Freeman 350/30 2,318,705 5/1943Morgan 350/15 3,450,480 6/1969 Chitayat 350/35 3,547,513 12/1970 Sheiner350/30 2,097,762 11/1937 Heine 350/15 2,255,631 9/1941 Schulman 350/142,128,394 8/1938 Berek 350/15 3,007,371 ll/l96l Tallman 350/15 3,620,59311/1971 Tackaberry 350/14 Primary ExaminerDavid H. Rubin Atto rneyKrafft& Wells and Gilbert L. Wells [57] ABSTRACT The split-field microscope ofthe present invention has a polarizer in the illuminator, a polarizingbeam splitter for simultaneous or alternative illumination of bothobject fields, equal optical path lengths for the polarized splitpartial beams between splitting and reunion,

- means for deflecting the polarized partial beams in the directiontoward parallel-disposed objectives and a quarter-wave plateldisposed ineach illuminating beam path. The imaging ray beams are'passed backwardsthrough the aforementioned components up to the central prism and a tubelens system and an ocular, connected after the central prism, aretransited by the combined imaging beams.

13 Claims, 7 Drawing Figures SPLIT-WELD MICROSCOPE BACKGROUND OF THEINVENTION The field of the invention is compound lens systems withplural optical'axes having a common axis portion with an axisinterceptor to shift from one axis to an other.

The present invention is particularly concerned with a split-fieldmicroscope wherein two objectives, each of which forms an image of thesame plane, have a variable spacing and have a common illuminatingdevice for both object fields.

These split-field microscopes are used to an increasing extent forcomparison and positioning purposes,

especially in semiconductor technology.

In this connection, it is important, for example, to exactly alignworking masks, emulsion masks, color masks, or chromium masks on siliconor germanium wafers. 7

In order tofulfrll these requirements, it is known to the prior art touse a mask adjusting microscope according to the principle of acomparison microscope wherein by using one objective at a time twopoints of the object are imaged side-by-side into respective viewinghalves of the ocular.

ln another prior art split-field microscope, a central prism is disposedto be displaceable so that either the left or the right object image isreproduced in the focal plane of the ocular. In the central position ofthese prisms, a left partial image and a right partial image are viewedsimultaneously. The illuminator is concomitantly moved in case ofchanges in distance of both objectives.

Finally, a split-field microscope with an asymmetrical beam path isknown from German Democratic Republic Pat. 'No. 62,166, wherein theillumination of both halves of the object is effected by a light sourceand a semi-transparent mirror. As a result of the asymmetrical course ofthe ray, the image of the aperture diaphragm of the illuminator assumesdifferent positions with respect to the pupils of v the two objectives.Thereby, the exit pupils behind the oculars appear to be differentsizes. Since the eye exhibits differing prop erties in the various zonesof the pupil, when it is observing a positioning procedure, thecongruence of a mask and semiconductor wafer is impaired, depending oneye imperfections. Besides, the vignetting in the fields can bedifferent.

A further disadvantage of this prior art split-field microscope residesin that 50 percent of the light is lost on the polarizers arranged onthe image side in front of the objectives. Furthermore, when apolarization anisotropy of the objects prevails, the object contrast ofboth objectives can be different.

Other prior art split-field microscopes exhibit the disadvantages that,on the one hand, they possess two or more illuminating lamps whichstrongly heat the device. On the other hand, there is no possibility forthe installation of cross hairs or marker diaphragms into the lightflux. Furthermore, only partial images'can be seenin the ocular; andfinally, merely an inaccurate, non-distrotion-free arrest of theobjectives set to a selected distance is possible.

SUMMARY OF THE INVENTION Having in mind the limitations of the priorart, it is an object of the present invention to provide an improvedsplit-field microscope for a broadened range of application.

This object is attained by providing a polarizer in the illuminator, apolarizing beam splitter for simultaneous or alternative illumination ofboth object fields, equal optical path lengths for the-polarized splitbeams between splitting and reunion, means for deflecting the polarizedpartial beams in the direction toward the parallel-disposed objectives,a quarter-wave plate disposed in each illuminating beam path, and bypassing the imaging ray beams backwards through the aforementionedcomponents up to the central prism, and connecting after the centralprism a tube lens system and an ocular transited by the combined imagingbeams.

In order to adapt the two objectives of the split-field microscope tothe size of the object to be examined, means are provided for varyingthe relative distances between the middle axis of the central prism andthe aiming axes of the objectives. In accordance with a further featureof the invention, each objective can be, respectively, one structuralunit with a beam-shifting component connected in front of the objectiveand a quarter-wave plate connected after the objective. Each structuralunit is, in each instance, disposed to be rotatable about the axis ofsymmetry of the polarized illuminating beam exiting from the respectivebeamdeflecting component, Besides, it is possible to fashion thesestructural units so thatthey can be inserted with varying orientations.Furthermore, the polarizer can be mounted to be rotatable, and theanalyzer can consist of two polarizing fields oriented at right anglesto each other.

In a further embodimennit is possible to provide an analyzer changerconsisting of an analyzer made up of two polarizing foils oriented atright angles to each other with respect to their directions oftransmissionandan analyzer made up of one polarizing foil. Furthermore,the illuminating device can comprise a light conductor, and areference'marker can be provided in the path of the illuminating beamwhich is conjugated with respect to the object planes. These imagesform, comparable to a protractor, .a standard distance which can befine-adjusted by means of an internal shifting lens.

. The tube lens system can consist of a combination of at least oneisotropic lens and a plane-parallel isotropic plate which can beinserted in and removed from the imaging beam path, or of a combinationof at least one isotropic lens and an anisotropic crystal plate, or of acombination of at least one isotropic lens and at least one anisotropiccrystal lens.

The advantages attained by the present invention reside particularly, inthat it is possible to obtain:

with only one illuminating device, a light efficiency which is improvedat least twofold;

a suppression of the disturbing scattered light produced in the opticalcomponents;

a possibility of installing cross-hairs into the beam path;

a sensitive distance adjustment of the aiming axes given by the nodalpoints of the objectives and the cross-hair of the ocular, with anextensive elimination of the errors produced by any inaccuracies inalignment;

equal path lengths for the polarized split beams between splitting andreunion;

an exact arresting of the objectives adjusted to the selected distance;

an imaging system with two axially different object planes; and

partial or mixed images of the object fields reproduced by bothobjectives.

BRIEF DESCRIPTION OF THE DRAWINGS DESCRIPTION OF THE PREFERREDEMBODIMENTS In FIG. 1, a beam of illuminating rays emanating from thelight source 1 and having an optical axis 1a impinges on the polarizer9, after passing through a field lens 2, a heat insulating filter 3, anda collimator the beam now penetrates this layer unhindered and isconducted to the ocular 23 via a tube lens system illustrated in thefigure by a lens 20 and a plano-parallel plate 21 disposed therebehind.In the observation beam path, an analyzer is furthermore arranged. Asshown in FIG. la, two different analyzers 31, 33, mounted on a commonanalyzer changer 22, are provided.

Let us now consider the case wherein the polarizer 9 is rotated by anangle of 90. Here, the linearly polarized beam of lightrays emanatingtherefrom exhibits such a plane of oscillation that it can passunhindered through the polarizing splitting layer 11 and passes, via apentaprism 24, a compensating prism 25, and a beam-deflecting rhomboidprism 26, to the second objective 27 with the quarter-wave plate 28connected thereafter and from there to the object 18 with aiming axis27a. The returning circularly polarized imaging beam again is convertedinto linearly polarized light with the plane of oscillation shifted by90, so that the beam is now reflected on the polarizing splitting layerin the direction toward the ocular.

.Thus, it is possible to observe, depending on the respective positionof the polarizer 9, either object details by way of the objective 16,or, after rotating the polarizer 9 by 90, object details only by way ofthe objective 27, or,with a rotation of the polarizer 9 by 45, objectdetails simultaneously with identical light intensity via both partialbeam paths. For this purpose,

4, via a light conductor 5 followed by an aperture disphragm 6 as wellas an illuminating field stop 7, and then via a further lens 8. Thepolarizer 9 is rotatably arranged. The linearly polarized light beamthen impinges on the polarizing splitting layer 11 of the central prism10. This polarizing splitting layer 11 has the property either totransmit partially or entirely, or to deflect partially or entirely, animpinging, linearly polarized beam, depending on the direction ofoscillation thereof relative to the principal direction of polarizationof the prism.

Assuming that the polarized beam of illuminating light is entirelyreflected on the polarizing splitting layer 11, the beam extends alongthe middle axis 12 of the central prism and, after a renewed reflection,is guided via a pentaprism 13 which deflects the linearly polarized beamof rays by 90, so that the beam now passes, via a displaceable lens 14and a rhomboid prism 15 having a ray-shifting effect, into the objective16 with an aiming axis 16a. The linearly polarized beam is circularlypolarized after passage through a quarter-wave plate 17 disposed at anangle of 45 between the direction of oscillation of the light and theprincipal direction of oscillation thereof, and impinges on the object18.

The beam of light reflected on the object is likewise circularlypolarized and again becomes a linearly polarized beam after passagethrough the quarter-wave plate, due to a renewed phase shift. However,the plane of oscillation of this light beam is changed by 90 withrespect to that of the illuminating light beam. The above describedstructural components are transited by this linearly polarized imagingbeam in the reverse direction up to the polarizing splitting layer 11.Due to the fact that the plane of oscillation is shifted by 90,

indexing devices, which are not shown, are provided at the polarizer 9for the exact setting of the polarizer into desired angular positions.

By the projection of the marker of a marker diaphragm insertable in theilluminating field stop 7, a fixed distance transferable to the objectsis given via the spacing of the aiming axes of the objectives.

The objective 16 can form a structural unit 29 with the quarter-waveplate 17 and the rhomboid prism 15 with the internal shifting lens 14.The components 26-28 can likewise constitute a structural unit 30.

In order to set the distance of the aiming axes given by the nodalpoints of the objectives and the marker, means are provided, which arenot shown, permitting the variation of the relative distances betweenthe components 24, 25; 10 and 13, and thus the structural units 29 and30 with respect to each other.

In this connection, equal optical path lengths for the polarized splitbeams between splitting and reunion are preserved.

Besides, the structural units 20 and 30, respectively,

can be disposed to be rotatable about the axes of the polarizedilluminating beam emanating from the pentaprism 13 and the compensatingprism 25, respectively. v

It is possible to employ, in place of devices for the rotation of thestructural units, other means making it possible to attach thesestructural units 29, 30 with varying orientation with respect to theobject plane.

As can be seen, the structural unit 29 comprises an internal shiftinglens 14 inserted therein, which lens makes it possible to subject thebeam path to a tilt for fine adjustment.

The unwanted reflected light components from the structural partsbetween the polarizing splitting layer 11 and the quarter-wave plates17, 28 are preserved in their direction of oscillation and thus are notconducted, by the polarizing splitting layer 11, into the ocular 23 viathe tube lens system 20, 21.

The optics of the tube lens system 26, 2t can be fashioned, as set forthhereinbelow, so that two different planes 1%, 19 in the object space canbe observed either simultaneously or successively in'a sharp image,without having to effect an additional focusing procedure at theobjectives.

If an isotropic plane-parallel plate 21 is inserted behind aconventional microlens system 20, the beam path is altered, along thelines that a different object plane is sharply reproduced. Once theplate 21 is again withdrawn, the original object plane is againreproduced in a sharp image. The plane-parallel plate 21 can alsoconsist of ananisotropic crystalline material, e.g. calcite or quartz,wherein the optical axis of the crystal is in each case parallel withrespect to the two plane surfaces. The linearly polarized light beamcoming from the polarizing splitting layer 11 impinges upon the crystalplate 21, cut in an oriented manner. This light beam having just leftthe lens system 20, is split into two linearly polarized components, dueto the anisotropy of the crystal and the planes of oscillation of theselast-mentioned components are at right angles to each other. In order tobe able to regulate the splitup components, caused by the anisotropicstructural elements, with respect to their partial image planes, thestructural elements are rotatably mounted.

The components, influenced by the two different indices of refraction nand n (n s n pass through the plate 211 at different velocities, so thata difference in path results therefrom which is dependent on thethickness of the plate 21.

If a normal analyzer'33 disposed on the analyzer changer 22 is insertedinto the beam path, this analyzer transmits only the beam portion, theplane of oscillation of which is in parallel to the transmissiondirection of the analyzer foil. Depending on the position in azimuth ofthe analyzer 33, the component being influenced by the index ofrefraction n or the component being influenced by the index ofrefraction n can thus be observed through the eyepiece. This means thatit is possible to focus selectively on the object plane 18 or the objectplane 19.

In contrast thereto, if the analyzer 31 is inserted into the beam path,which analyzer consists of two .analyzer foils which are at right anglesto each other with respect to their directions of transmission,respectively one foil half allows respectively one linearly polarizedbeam component to pass through so that, with only one focal adjustment,two differing object planes can be simultaneously sharply reproduced.This is particularly advantageous in positioning procedures insemiconductor technology. I

It is self-evident that also a binocular can take the place of theocular 23. However, in this case, the analyzer changer is suitablyarranged in front of the splitting prism of the binocular.

Also, objective pairs of differing magnification can be employed whichcan be mounted, for example, on

slides or turrets.

It is also possible to provide means for the continuous rotation of thepolarizer 9. Thereby, a chronologically rapidly changing imaging of theleft and right object fields is achieved. In the case where therotational frequency is adjusted to the flicker frequency of the eye ofthe observer, image details which are different on the two object halvesare emphasized. However, this leads to further possibilities ofapplication for the 6 novel split-field microscope, for example as acomparison microscope for criminological investigations, as anerror-tracking device, a color comparison device, and other uses.

FIG. 2 shows the same apparatus, wherein the objectives are disposed asclosely together as possible. For this purpose, the structural units 29,30 were rotated by and the structural unit 29 was moved, together withthe pentaprism 13, in the direction toward the central prism 10, and thestructural unit 30 was moved together with the pentaprism 24 and thecompensating prism 25 likewise in the same direction.

In FIGS. 3-6, additional embodiments are illustrated,

wherein several details included in FIGS. 1 and 2 have been omitted forthe sake of simplicity.

' In FIG. 3, the pentaprism 13 is combined with the compensating prism34 and the objective 116 with quarter-wave plate 117 into a fixedstructural unit 36. The pentaprism 24 is combined with the compensatingprism 25 and the objective '27 with the quarter-wave plate 28 into astructural unit 35. Both structural units 35, 36 are arranged to bedisplaceable with respect to the middle axis 12 of the central prism 10,with the beampath lengths being identical.

In FIG. 4, deviating prisms 37, 39 are disposed fixedly at the device,and the variation of the relative distances of the objectives 16, 27 iseffected by shifting the structural elements 40, l6, l7 and 38, 27, 28respectively, which are combined into the structural units 4l1l and 42respectively.

In FIG. 5, an arrangement is shown wherein, in a double prism 43, thepolarizing splitting layer 1 l is arranged verticallyf Thebeam-deflecting components consist of a pentaprism 24 and a deviatingprism 50 with compensating plates 46, 47 cemented thereon. Thestructural elements, combined into units 48, 49 are arranged to bedisplaceable in the usual manner. ""xsatnfiaaantisifiaieated in FIG. 6.Here, the double prism, provided with a polarizing splitting layer 11,is mounted to be slidable to and fro with respect to the fixedilluminating beam axis 54 and the fixed tube axis 5 5. By means coupledmechanically with the reciprocating motion of the prism 43, the elements50, 27, 26 on the one hand, and 51, 16, 17, on the other hand, combinedinto structural units 52,and 53, are shifted, in correspondence with thespreading of the lasa na H We claim:

a. said beam splitter comprising a polarizing beam splitter in saidcentral prism along said optical axis for the simultaneous oralternative illumination of both said object'fields'by means of arotatable polarizer;

b. first means a first polarized portion of said illuminating beam inthe direction of said first objective;

c. second means deflecting a second polarized portion of saidilluminating beam in a direction toward said second ojbective;

d. a first quarter-wave plate disposed in said first polarized portionbetween said polarizing beam splitter and a first object field throughwhich said illuminating beam passes to the first object field and afirst imaging partial beam passes backwards;

. a second quarter-wave plate disposed in said second polarized portionbetween said polarizing beam splitter and a second object field throughwhich said illuminating beam passes to the second object field and asecond imaging partial beam passes backwards; and

f. an image-producing optical system connected between said polarizingbeam splitter and said eyepiece in the direction of said first andsecond imaging partial beams, said optical system containingplano-parallel optical elements of an isotropic material, said elementsremovable from the beam path, said optical system reproducing saidimages of said object fields of the said same plane, axially offset onthe viewing side.

2. In a split-field microscope having a central prism with a beamsplitter, a first objective with a first aiming axis, a second paralleldisposed objective with a second aiming axis, a common illuminatingdevice producing an illuminating beam along an optical axis for bothobject fields, said objectives having a variable spacing of theiroptical axes, both forming images of said object fields, which arecontained in the same plane, and an eyepiece, the improvement comprisngthe combination of:

a. said beam splitter comprising a polarizing beam splitter in saidcentral prism along said optical axis for the simultaneous oralternative illumination of both said object fields by means of arotatable polarizer;

b. first means deflecting a first polarized portion of said illuminatingbeam inthe direction of said first objective;

c. second means deflecting a second polarized portion of saidilluminating beam in a direction toward said second objective;

d. a first quarter-wave plate disposed in said first plarized portionbetween said polarizing beam splitter and a first object field throughwhich said illuminating beam passes to the first object field and afirst imaging partial beam passes backwards;

e. a second quarter-wave plate disposed in said second polarized portionbetween said polarizing beam splitter and a second object field throughwhich said illuminating beam passes to the second object field and asecondimaging partial beam passes backwards; and

f. an image producing optical system connected between said polarizingbeam splitter and said eyepiece in the direction of said first andsecond imaging partial beams, said optical system containingplano-parallel optical elements of an anisotropic material, saidelements rotatable in the beam path, said optical system reproducingsaid images of said object fields of said same plane, axially offset onthe viewing side.

3. In a split-field microscope having a central prism with a beamsplitter, a first objective with a first aiming axis, a second paralleldisposed objective with a second aiming axis, a common illuminatingdevice producing an illuminating beam along an optical axis for bothobject fields, said objectives having a variable spacing of theiroptical axes, both forming images of said object fields, which arecontained in the same plane, and an eyepiece, the improvement comprisingthe combination of:

a. said beam splitter comprising a polarizing beam splitter in saidcentral prism along said optical axis for the simultaneous oralternative illumination of both said object fields by means of arotatable polarizer;

b. first means deflecting a first polarized portion of said illuminatingbeam in the direction of said first objective;

0. second means deflecting a second polarized portion of saidilluminating beam in a direction toward said second objective;

(1. a first quarter-wave plate disposed in said first polarized portionbetween said polarizing beam splitter and a first object field throughwhich said illuminating beam passes to the first object field and afirst imaging partial beam passes backwards;

, a second quarter-wave plate disposed in said second polarized portionbetween said polarizing beam splitter and a second object field throughwhich said illuminating beam passes to the second object field and asecond imaging partial beam passes backwards; and v f. animage-producing optical system connected between said polarizing beamsplitter and said eyepiece in the direction of said first and secondimaging partial beams, said optical system containing a plane-paralleloptical element of an anisotropic material and an optical lens of anisotropic material, said plano-parallel optical element rotatable in thebeam path, said optical system reproducing said images of said objectfields of the said same plane, axially offset on the viewing side.

4. In a split-field microscope having a central prism with a beamsplitter, a first objective with a first aiming axis, a second paralleldisposed objective with a second aiming axis, a common illuminatingdevice producing an illuminating beam along an optical axis for bothobject fields, said objectives having a variable spacing of theiroptical axes, both forming images of said object fields, which arecontained in the same plane, and an eyepiece, the improvement comprisingthe combination of:

a. said beam splitter comprisng a polarizing beam splitter in saidcentral prism along said optical axis for the simultaneous oralternative illumination of both said object fields by means of arotatable polarizer;

b. first means deflecting a first polarized portion of said illuminatingbeam in the direction of said first objective;

c. second means deflecting a second polarized portion of saidilluminating beam in a direction toward said second objective;

d. a first quarter-wave plate disposed in said first polarized portionbetween said polarizing beam split-' ter and a first object fieldthrough which said illuminating beam passes to the first object fieldand a first imaging partial beam passes backwards;

e. a second quarter-wave plate disposed in said second polarized portionbetween said polarizing beam splitter and a second object field throughwhich said illuminating beam passes to the second object field and asecond imaging partial beam passes backwards; and an image-producingoptical system connected between said polarizing beam splitter and saideyepiece in the direction of said first and second imaging partialbeams, said optical system containing an optical lens of an anisotropicmaterial, said optical lens rotatable in the beam path, said opticalsystem reproducing said images of said object fields of the said sameplane, axially offset on the viewing side. l 5. in a split-fieldmicroscope having a central prism with a beam splitter, a firstobjective with a first aiming axis, a second parallel disposed objectivewith a second aiming axis, a common illuminating device producing anilluminating beam along an optical axis for both object fields, saidobjectives having a variable spacing of their optical axes, both formingimages of said object fields, which are contained in the same plane, andan eyepiece, the improvement comprising the combination of: I

a. said beam splitter comprising a polarizing beam splitter in saidcentral prism along said optical axis for the simultaneous oralternative illumination of both said object fields by means of arotatable polarizer;

b. first means deflecting a first polarized portion of said illuminatingbeam in the direction of said first objective;

c. second means deflecting a second polarized portion of saidilluminating beam in a direction toward said second objective;

d. a first quareter-wave plate disposed in said first polarized portionbetween said polarizing beam splitter and a first object field throughwhich said illuminating beam passes to the first object field and afirst imaging partial beam passes backwards;

. a second quarter-wave plate disposed in said second polarized portionbetween 'said polarizing beam splitter and a second object field throughwhich said illuminating beam passes backwards; and an image-producingoptical system connected between said polarizi'ng'beam splitter and saideyepiece in the direction of said first and second imaging partialbeams, said optical system containing a plane-parallel isotropic plateand an isotropic lens, said plano-parallel isotropic plate removablefrom the beam path, said optical system reproducing said images of saidobject fields of the same plane, axially offset on the viewing. side.

6. ln-a split-field microscope having a central prism with a beamsplitter, a first objective with a first aiming axis, a second paralleldisposed objective with a second aiming axis, a common illuminatingdevice producing an illuminating beam along an optical axis for bothobject fields, said objectives having a variable spacing of theiroptical axes, both forming images of said object fields, which arecontained in the same plane, and an eyepiece, the improvement comprisingthe combination of:

a. said beam splitter comprising a polarizing beam splitter in saidcentral prism along said optical axis forthe simultaneous or alternativeillumination of both said object fields by means of a rotatablepolarizer;

b. first means deflecting a first polarized portion of said illuminatingbeam in the direction of said first objective;

c. second means deflecting a second polarized portion of saidilluminating beam in a direction toward said second objective;

d. a first quarter-wave plate disposed in said first p0- larized portionbetween said polarizing beam splitter and a first object field throughwhich said illuminating beam passes to the first object field and afirst imaging partial beam passes backwards;

e. a second quarter-wave plate disposed in said second polarized portionbetween said polarizing beam splitter and a second object field throughwhich said illuminating beam passes to the second object field and asecond imaging partial beam passes backwards; and

f. an image-producing optical system connected between said polarizingbeam splitter and said eyepiece in the direction of said first andsecond imaging partial beams, said optical system containing at leastone isotropic lens and an anisotropic crystalline lens, said isotropiclens removable from the beam path and said anisotropic crystalline lensrotatable in the beam path, said optical system reproducing said imagesof said object fields of the same plane, axially offset on the viewingside.

7. The split-field microscope according to claim 1, wherein saideyepiece has two lenses in spaced relationship along said optical axisand a linearly polarizing optical element is provided along said opticalaxis after said polarizing beam splitter, between said two lenses.

8. The split-field microscope accordirig to claim '7 wherein saidpolarizing optical element comprises at least two polarizing foilsarranged side by side and oriented at right angles to each other withrespect to their transmission directions.

fiTfie split-fieldirlfirosc756according to claim 7, wherein a device forchanging of the said polarizing optical element is provided comprising afirst polarizing optical element consisting of two polarizing foilsarranged side by side and oriented at right angles to each other withrespect to their transmission directions and a second polarizing opticalelement consisting of one polarizing foil. illi Th7; spiitr'ii'ifilc'fbscdpe accoridng to claim 7, wherein a drive is provided forthe continuous rotation of the polarizing optical component.

ill. The split-field microscope accordirig to cEim 1, wherein means areprovided for varying the relative distances between the central axis ofsaid central prism containing said polarizing beam splitter and saidaiming axes of the objectives, said means for varying comprising atfi'st striicturafinit; wlfiilicontains a shifting lens,

a first beam-bending component, said first objective and said firstquarter-wave plate, means for rotating by said first structural unittogether with a first pentaprism, said pentaprism arranged between saidcentral prism and said first structural unit, in the direction towardsaid central prism,

and a second structural unitfwhich contains a second beam-bendingcomponent, said second objective and said second quarter-wave plate,means for rotating by 180 said second structural unit together with asecond pentaprism, in the direction toward said central prism, saidsecond pentaprism connected with a compensating prism and arrangedbetween said central prism and said second structural unit. 12. Thesplit-field microscope according to claim 1, wherein each objectiveforms, together with a for- 1 wardly associated beam-bending componentand a quarter-wave plateconnected thereafter, a structural an aperturediaphragm and a lens of said objective.

1. In a split-field microscope having a central prism with a beamsplitter, a first objective with a first aiming axis, a second paralleldisposed objective with a second aiming axis, a common illuminatingdevice producing an illuminating beam along an optical axis for bothobject fields, said objectives having a variable spacing of theiroptical axes, both forming images of said object fields, which arecontained in the same plane, and an eyepiece, the improvement comprisingthe combination of: a. said beam splitter comprising a polarizing beamsplitter in said central prism along said optical axis for thesimultaneous or alternative illumination of both said object fields bymeans of a rotatable polarizer; b. first means deflecting a firstpolarized portion of said illuminating beam in the direction of saidfirst objective; c. second means deflecting a second polarized portionof said illuminating beam in a direction toward said second objective;d. a first quarter-wave plate disposed in said first polarized portionbetween said polarizing beam splitter and a first object field throughwhich said illuminating beam passes to the first object field and afirst imaging partial beam passes backwards; e. a second quarter-waveplate disposed in said second polarized portion between said polarizingbeam splitter and a second object field through which said illuminatingbeam passes to the second object field and a second imaging partial beampasses backwards; and f. an image-producing optical system connectedbetween said polarizing beam splitter and said eyepiece in the directionof said first and second imaging partial beams, said optical systemcontaining plano-parallel optical elements of an isotropic material,said elements removable from the beam path, said optical systemreproducing said images of said object fields of the said same plane,axially offset on the viewing side.
 2. In a split-field microscopehaving a central prism with a beam splitter, a first objective with afirst aiming axis, a second parallel disposed objective with a secondaiming axis, a common illuminating device producing an illuminating beamalong an optical axis for both object fields, said objectives having avariable spacing of their optical axes, both forming images of saidobject fields, which are contained in the same plane, and an eyepiece,the improvement comprising the combination of: a. said beam splittercomprising a polarizing beam splitter in said central prism along saidoptical axis for the simultaneous or alternative illumination of bothsaid object fields by means of a rotatable polarizer; b. first meansdeflecting a first polarized portion of said illuminating beam in thedirection of said first objective; c. second means deflecting a secondpolarized portion of said illuminating beam in a direction toward saidsecond objective; d. a first quarter-wave plate disposed in said firstpolarized portion between said polarizing beam splitter and a firstobject field through which said illuminating beam passes to the firstobject field and a first imaging partial beam passes backwards; e. asecond quarter-wave plate disposed in said second polarized portionbetween said polarizing beam splitter and a second object field throughwhich said illuminating beam passes to the second object field and asecond imaging partial beam passes backwards; and f. an image producingoptical system connected between said polarizing beam splitter and saideyepiece in the direction of said fiRst and second imaging partialbeams, said optical system containing plano-parallel optical elements ofan anisotropic material, said elements rotatable in the beam path, saidoptical system reproducing said images of said object fields of the saidsame plane, axially offset on the viewing side.
 3. In a split-fieldmicroscope having a central prism with a beam splitter, a firstobjective with a first aiming axis, a second parallel disposed objectivewith a second aiming axis, a common illuminating device producing anilluminating beam along an optical axis for both object fields, saidobjectives having a variable spacing of their optical axes, both formingimages of said object fields, which are contained in the same plane, andan eyepiece, the improvement comprising the combination of: a. said beamsplitter comprising a polarizing beam splitter in said central prismalong said optical axis for the simultaneous or alternative illuminationof both said object fields by means of a rotatable polarizer; b. firstmeans deflecting a first polarized portion of said illuminating beam inthe direction of said first objective; c. second means deflecting asecond polarized portion of said illuminating beam in a direction towardsaid second objective; d. a first quarter-wave plate disposed in saidfirst polarized portion between said polarizing beam splitter and afirst object field through which said illuminating beam passes to thefirst object field and a first imaging partial beam passes backwards; e.a second quarter-wave plate disposed in said second polarized portionbetween said polarizing beam splitter and a second object field throughwhich said illuminating beam passes to the second object field and asecond imaging partial beam passes backwards; and f. an image-producingoptical system connected between said polarizing beam splitter and saideyepiece in the direction of said first and second imaging partialbeams, said optical system containing a plano-parallel optical elementof an anisotropic material and an optical lens of an isotropic material,said plano-parallel optical element rotatable in the beam path, saidoptical system reproducing said images of said object fields of the saidsame plane, axially offset on the viewing side.
 4. In a split-fieldmicroscope having a central prism with a beam splitter, a firstobjective with a first aiming axis, a second parallel disposed objectivewith a second aiming axis, a common illuminating device producing anilluminating beam along an optical axis for both object fields, saidobjectives having a variable spacing of their optical axes, both formingimages of said object fields, which are contained in the same plane, andan eyepiece, the improvement comprising the combination of: a. said beamsplitter comprising a polarizing beam splitter in said central prismalong said optical axis for the simultaneous or alternative illuminationof both said object fields by means of a rotatable polarizer; b. firstmeans deflecting a first polarized portion of said illuminating beam inthe direction of said first objective; c. second means deflecting asecond polarized portion of said illuminating beam in a direction towardsaid second objective; d. a first quarter-wave plate disposed in saidfirst polarized portion between said polarizing beam splitter and afirst object field through which said illuminating beam passes to thefirst object field and a first imaging partial beam passes backwards; e.a second quarter-wave plate disposed in said second polarized portionbetween said polarizing beam splitter and a second object field throughwhich said illuminating beam passes to the second object field and asecond imaging partial beam passes backwards; and f. an image-producingoptical system connected between said polarizing beam splitter and saideyepiece in the direction of said first and second imaging partialbeams, said optical system containing an optical lens of an anisotropicmAterial, said optical lens rotatable in the beam path, said opticalsystem reproducing said images of said object fields of the said sameplane, axially offset on the viewing side.
 5. In a split-fieldmicroscope having a central prism with a beam splitter, a firstobjective with a first aiming axis, a second parallel disposed objectivewith a second aiming axis, a common illuminating device producing anilluminating beam along an optical axis for both object fields, saidobjectives having a variable spacing of their optical axes, both formingimages of said object fields, which are contained in the same plane, andan eyepiece, the improvement comprising the combination of: a. said beamsplitter comprising a polarizing beam splitter in said central prismalong said optical axis for the simultaneous or alternative illuminationof both said object fields by means of a rotatable polarizer; b. firstmeans deflecting a first polarized portion of said illuminating beam inthe direction of said first objective; c. second means deflecting asecond polarized portion of said illuminating beam in a direction towardsaid second objective; d. a first quarter-wave plate disposed in saidfirst polarized portion between said polarizing beam splitter and afirst object field through which said illuminating beam passes to thefirst object field and a first imaging partial beam passes backwards; e.a second quarter-wave plate disposed in said second polarized portionbetween said polarizing beam splitter and a second object field throughwhich said illuminating beam passes backwards; and f. an image-producingoptical system connected between said polarizing beam splitter and saideyepiece in the direction of said first and second imaging partialbeams, said optical system containing a plano-parallel isotropic plateand an isotropic lens, said plano-parallel isotropic plate removablefrom the beam path, said optical system reproducing said images of saidobject fields of the same plane, axially offset on the viewing side. 6.In a split-field microscope having a central prism with a beam splitter,a first objective with a first aiming axis, a second parallel disposedobjective with a second aiming axis, a common illuminating deviceproducing an illuminating beam along an optical axis for both objectfields, said objectives having a variable spacing of their optical axes,both forming images of said object fields, which are contained in thesame plane, and an eyepiece, the improvement comprising the combinationof: a. said beam splitter comprising a polarizing beam splitter in saidcentral prism along said optical axis for the simultaneous oralternative illumination of both said object fields by means of arotatable polarizer; b. first means deflecting a first polarized portionof said illuminating beam in the direction of said first objective; c.second means deflecting a second polarized portion of said illuminatingbeam in a direction toward said second objective; d. a firstquarter-wave plate disposed in said first polarized portion between saidpolarizing beam splitter and a first object field through which saidilluminating beam passes to the first object field and a first imagingpartial beam passes backwards; e. a second quarter-wave plate disposedin said second polarized portion between said polarizing beam splitterand a second object field through which said illuminating beam passes tothe second object field and a second imaging partial beam passesbackwards; and f. an image-producing optical system connected betweensaid polarizing beam splitter and said eyepiece in the direction of saidfirst and second imaging partial beams, said optical system containingat least one isotropic lens and an anisotropic crystalline lens, saidisotropic lens removable from the beam path and said anisotropiccrystalline lens rotatable in the beam path, said optical systemreproducing said images of said object fields of the same plane, Axiallyoffset on the viewing side.
 7. The split-field microscope according toclaim 1, wherein said eyepiece has two lenses in spaced relationshipalong said optical axis and a linearly polarizing optical element isprovided along said optical axis after said polarizing beam splitter,between said two lenses.
 8. The split-field microscope according toclaim 7, wherein said polarizing optical element comprises at least twopolarizing foils arranged side by side and oriented at right angles toeach other with respect to their transmission directions.
 9. Thesplit-field microscope according to claim 7, wherein a device forchanging of the said polarizing optical element is provided comprising afirst polarizing optical element consisting of two polarizing foilsarranged side by side and oriented at right angles to each other withrespect to their transmission directions and a second polarizing opticalelement consisting of one polarizing foil.
 10. The split-fieldmicroscope according to claim 7, wherein a drive is provided for thecontinuous rotation of the polarizing optical component.
 11. Thesplit-field microscope according to claim 1, wherein means are providedfor varying the relative distances between the central axis of saidcentral prism containing said polarizing beam splitter and said aimingaxes of the objectives, said means for varying comprising a firststructural unit, which contains a shifting lens, a first beam-bendingcomponent, said first objective and said first quarter-wave plate, meansfor rotating by 180* said first structural unit together with a firstpentaprism, said pentaprism arranged between said central prism and saidfirst structural unit, in the direction toward said central prism, and asecond structural unit, which contains a second beam-bending component,said second objective and said second quarter-wave plate, means forrotating by 180* said second structural unit together with a secondpentaprism, in the direction toward said central prism, said secondpentaprism connected with a compensating prism and arranged between saidcentral prism and said second structural unit.
 12. The split-fieldmicroscope according to claim 1, wherein each objective forms, togetherwith a forwardly associated beam-bending component and a quarter-waveplate connected thereafter, a structural unit; and each of said twostructural units is disposed with variable orientation about the centralaxis of the polarized illuminating beam leaving the respectivebeam-bending component.
 13. The split-field microscope according toclaim 1, wherein a marker diaphragm is provided in the illuminating beampath, said marker diaphragm is insertable in an illuminating field stop,which is arranged between an aperture diaphragm and a lens of saidobjective.